THE RATIONALE FOR A LONG-LIVED GEOPHYSICAL NETWORK MISSIONTO MARS
Submitted to
The Mars Panel, NRC Decadal Survey for the Planetary Sciences Division, SMD, NASA
Phil Christensen, Chair; Wendy Calvin, Vice Chair
Written by
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Bruce Banerdt JPL
Tilman Spohn DLR
Ulli Christensen MPS
Veronique Dehant ROB
Linda Elkins-Tanton MIT
Robert Grimm SwRI
Matthias Grott DLR
Bob Haberle NASA-Ames
Martin Knapmeyer DLR
Philippe Lognonné IPGP
Franck Montmessin
Service Aeronomie
Yosio Nakamura
U Texas (ret.)
Roger Phillips SwRI
Scot Rafkin SwRI
Peter Read Oxford
Gerald Schubert UCLA
Sue Smrekar JPL
Mike Wilson JPL
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Endorsed by
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Oded Aharonson Caltech
Don Albert CRREL
Carlton Allen NASA-JSC
Robert Anderson JPL
Scott Anderson SwRI
Jeff Andrews-Hanna
Colo. School of Mines
Jafar Arkani-Hamed McGill U
Gabriele Arnold U Münster
Sami Asmar JPL
Lisa Baldwin DLR
Don Banfield Cornell
Amy Barr SwRI
Connie Bertka Carnegie
Jens Biele DLR
Bruce Bills JPL
Joachim Block DLR
Lars Borg LLNL
Ute Böttger DLR
Neil Bowles Oxford
Doris Breuer DLR
Nathan Bridges APL
Simon Calcutt Oxford
David Catling
U Washington
Eric Chassefière SA
Valerie Ciarletti
IPSL-LATMOS
Agustin Chicarro
ESA-ESTEC
Eric Clévédé IPGP
Barbara Cohen MSFC
David Crisp JPL
Paul Davis UCLA
Greg Delory UC Berkeley
Jean-Pierre de Vera DLR
Alan Delamere DSS
Cynthia Dinwiddie SwRI
James Dohm U Arizona
Andrew Dombard
U Illinois
Jim Dorman CERI
Bethany Ehlmann Brown
Richard Elphic
NASA-Ames
Yingwei Fei Carnegie
Eric Fielding JPL
Justin Filoberto LPI
Bill Folkner JPL
François Forget
CNRS-LMD
Matt Fouch ASU
Brenda Franklin JPL
Herbert Frey GSFC
Jeannine Gagnepain-Beyneix IPGP
Rafael Garcia
Obs. Midi-Pyrenees
Jim Garvin GSFC
Rebecca Ghent U Toronto
Domenico Giardini ETH
Lori Glaze GSFC
Matthew Golombek JPL
Natalia Gómez Pérez Carnegie
John Grant Smithsonian
Eric Grosfils
Pomona College
Albert Haldemann
ESA-ESTEC
Vicky Hamilton SwRI
Ari-Matti Harri FMI
Ernst Hauber DLR
Steve Hauck Case Western
James Head III Brown
Michael Hecht JPL
Robert Herrick U Alaska
Noel Hinners Consultant
Harald Hoffmann DLR
Lon Hood U Arizona
Shaopeng Huang
U Michigan
Troy Hudson JPL
Joel Hurowitz JPL
Hauke Hussmann DLR
Brian Hynek U Colorado
Anton Ivanov PSI
Erik Ivins JPL
Ralf Jaumann DLR
Catherine Johnson
U British Columbia
Donna Jurdy Northwestern
Jeff Kargel U Arizona
Günter Kargl
Austrian Acad. Sci.
Sharon Kedar JPL
Amir Khan U Copenhagen
Krishan Khurana UCLA
Walter Kiefer LPI
Scott King Virginia Tech
Kurt Klaus Boeing
Jörg Knollenberg DLR
Naoki Kobayashi JAXA
Ulrich Koehler DLR
Carlos Lange U Alberta
Gary Latham DOE (ret.)
Mark Leese Open U
Frank Lemoine GSFC
Robert Lillis UC Berkeley
John Longhi
Lamont-Doherty
Paul Lundgren JPL
Mioara Mandea GFZ
Michael Manga
UC Berkeley
Guy Masters UCSD
Pat McGovern LPI
Dan McKenzie Cambridge
Daniel Mège U Nantes
Michel Menvielle IPSL/LATMOS
Jon Merrison U Aarhus
Colleen Milbury UCLA
David Mimoun U Toulouse
Antoine Mocquet U Nantes
Dirk Möhlmann DLR
Jean-Paul Montagner IPGP
Laurent Montesi
U Maryland
William Moore UCLA
Paul Morgan NAU
Seiichi Nagihara
Texas Tech
Clive Neal U Notre Dame
William Newman UCLA
Horton Newsom UNM
Francis Nimmo UCSC
Daniel Nunes JPL
Jürgen Oberst DLR
Emile Okal Northwestern U
Dimitri Papanastassiou JPL
Marc Parmentier Brown
Manish Patel Open U
Tom Pike Imperial College
Jeffrey Plescia APL
Michael Purucker Raytheon-GSFC
Jouko Raitala U Oulu
Carol Raymond JPL
Lutz Richter DLR
Pascal Rosenblatt ROB
Thomas Ruedas Carnegie
Chris Russell UCLA
David Sandwell Scripps
Nicholas Schmerr Carnegie
Nicole Schmitz DLR
Richard Schultz U Nevada
Mindi Searls U Colorado
Karsten Seiferlin U Bern
Nikolai Shapiro IPGP
Charles Shearer UNM
Brian Shiro NOAA
Mark Simons Caltech
Norman Sleep Stanford
John C. Smith Petro-Frac
Frank Sohl DLR
Slava Solomatov Washington U
Christophe Sotin JPL
Aymeric Spiga Open U
Sabine Stanley U Toronto
Bernhard Steinberger GFZ
Bob Strangeway UCLA
Elénore Stutzmann IPGP
Seiji Sugita U Tokyo
Paul Tackley ETH
Satoshi Tanaka JAXA
Jeff Taylor U Hawaii
Ross Taylor ANU
Nicholas Teanby Oxford
Nafi Toksöz MIT
Allan Treiman LPI
Jeroen Tromp Princeton
Stephan Ulamec DLR
Tim Van Hoolst ROB
Olivier Verhoeven
U Nantes
Heinrich Villinger
U Bremen
Tom Watters Smithsonian
Wesley Watters Cornell
Renee Weber USGS
Mark Wieczorek IPGP
Jonathan Weinberg
Ball Aerospace
Ben Weiss MIT
David Williams ASU
Rebecca Williams PSI
Colin Wilson Oxford
Greg Wilson JPL
Shijie Zhong U Colorado
Maria Zuber MIT
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Introduction
The signatories of this paper support the development of a set of Mars surface stations (a“network”) to study interior geophysical and surface meteorological science. These stations would provide continuous, high frequency measurements not possible from orbit. The science objectives for a Mars geophysical network have been consistently highly recommended by the National Academy for the past 30 years (see National Research Council, 1978, 1988, 1990, 1994, 1996, 1997,2003a,b, 2008). In particular, the science from this network would directly address many of the previous decadal survey themes (National Research Council, 2003b), along with their attendant measurements:
1. What led to the unique character of our home planet (the past)?
- Interior and bulk planetary composition
- Internal structure and evolution
- Horizontal and vertical variations in internal structure and composition
- Major heat-loss mechanisms
- Major characteristics of the iron-rich metallic core
2. What common dynamic processes shape Earth-like planets (the present)?
- Processes that stabilize climate
- Processes and rates of surface/atmosphere interaction
- Active internal processes that shape atmospheres and surfaces
- Current volcanic and/or tectonic activity
3. What fate awaits Earth’s environment and those of the other terrestrial planets (the future)?
- Consequences of impacting particles and large objects
- Current flux of impactors
Note that the objectives and methods described in this white paper have significant overlap with a number of other white papers submitted to the Decadal Survey (e.g., Asmar et al., 2009; Edwards et al., 2009;Grimm, 2009; Lillis et al., 2009; MEPAG, 2009a,b; Mischna et al., 2009; Rafkin et al., 2009; Ruedas et al., 2009), illustrating the broad applicability to planetary science.
In the following sections we will outline the scientific rationale for a network mission to Mars, described the measurements required, and summarize key features of its implementation.
The Scientific Value of MarsInterior Investigationsfrom Surface-Based Geophysics
Our fundamental understanding of the interior of the Earth comes from geophysics, geochemistry, and petrology. For geophysics, seismology, together with surface heat flow, magnetic, paleomagnetic and gravity field measurements, and electromagnetic (EM) techniques, have revealed the basic internal layering of the Earth, its thermal structure, its gross compositional stratification, as well as significant lateral variations in these quantities. For example, seismological, magnetic and paleomagnetic measurements revealed the basic components of seafloor spreading and subduction, and seismology alone has mapped the structure of the core, compositional and phase changes in the mantle, three-dimensional velocity anomalies in the mantle related to subsolidus convection, and lateral variations in lithospheric structure. Additionally, seismic information placed strong constraints on Earth’s interior temperature distribution and the mechanisms of geodynamo operation. The comprehension of how life developed and evolved on Earth requires knowledge of Earth’s thermal and volatile evolution and how mantle and crustal heat transfer, coupled with volatile release, affected habitability at and near the planet’s surface. Whereas geophysics can provide information about past processes and states required to reach this understanding, it primarily provides a “snapshot” at one instant in time of how the Earth behaves. This “boundary condition” is a powerful constraint on all models that describe the history of the Earth and attempt to place the evolution of life in this framework, as such models must evolve to this present state.
Mars is a counterpoint to the Earth in how a terrestrial planet evolves. Earth’s thermal engine has transferred heat to the surface largely by lithospheric recycling over much of its history, but on Mars there is no evidence in the available record that this process ever occurred (e.g., Pruis and Tanaka, 1995; Sleep and Tanaka, 1995). Over the past ~4 billion years, giant hotspots (Tharsis and Elysium) have played a significant role in the tectonic and thermal evolution of the planet, and possibly had a causal relationship to an early core dynamo (Golombek and Phillips, 2009), which may, in turn, have been crucial for shielding Mars’ early atmosphere from solar wind erosion (Fang et al., 2009). Furthermore, these volcanic complexes released massive amounts of volatiles to the martian atmosphere, which possibly led to clement conditions at times and provided favorable habitability environments (Phillips et al., 2001).
Although the Earth has lost the structures caused by differentiation and early evolution because of vigorous mantle convection, Mars may retain evidence, such as azimuthal and radial compositional differentiation in the crust and mantle.Martian meteorite compositions indicate melting source regions with different compositions that have persisted since the earliest evolution of the planet (Jones, 1986; Borg et al., 1997, 2002), suggesting that mantle convection has been insufficiently vigorous to homogenize the mantle. Further, much of the martian crust dates to the first half billion years of the solar system (Frey et al., 2002). Measurements of the planetary interior may therefore detect structures that still reflect differentiation and early planetary formation processes, making Mars an ideal subject for geophysical investigations aimed at understanding planetary accretion and early evolution.Accretion without initial melting, however, may produce earlier, more vigorous convection, which would have eliminated azimuthal compositional variations (Schubert and Spohn, 1990).
Planetary interiors not only record evidence of conditions of planetary accretion and differentiation, they exert significant control on surface environments. The structure of a planetary interior and its dynamics control heat transfer within a planet through advected mantle material, heat conducted through the lithosphere, and volcanism. Volcanism in particular controls the timing of volatile release, and influences the availability of water and carbon. The existence and strength of any planetary magnetic field depends in part upon the size and state of the core.
The crust of a planet is generally thought to form initially through fractionation of an early magma ocean, with later addition through partial melting of the mantle and resulting volcanism. Thus the volume (thickness) and structure of the crust places strong constraints on the depth and evolution of the putative martian magma ocean and, by extension, planetary magma oceans in general. Currently we do not know the volume of Mars’ crust to within a factor of two. Orbital data allows the calculations of variations of crustal thickness (Neumann et al., 2004), but models generally must assume a mean thickness and uniform density for lack of any constraints.
Knowledge of the state of Mars’ core and its size is important for understanding the planet’s evolution. The thermal evolution of a terrestrial planet can be deduced from the dynamics of its mantle and core. The evolution of a planet and the possibility of dynamo magnetic field generation in its core are highly dependent on the planet’s ability to develop convection in the core and in the mantle. In particular, a core magneto-dynamo is related either to a high thermal gradient in the liquid core (thermally driven dynamo) or to the growth of a solid inner core (chemically driven dynamo), or both (Longhi et al., 1992; Dehant et al., 2007, 2009; Breuer et al., 2007). The state of the core depends on the percentage of light elements in the core and on the core temperature, which is related to the heat transport in the mantle(Stevenson, 2001;Breuer and Spohn, 2003, 2006; Schumacher and Breuer, 2006). Thus the present size and state of the core has important implications for our understanding of the evolution and present state of Mars (Dehant et al., 2007, 2009;Stevenson, 2001;Breuer et al., 1997; Spohn et al., 2001; Van Thienen et al., 2007), yet the value of its radius is uncertain to ±10% and it is unclear whether it is solid, liquid or both.
Mantle dynamics plays a key role in shaping the geology of the surface through volcanism and tectonics(Van Thienen et al., 2007). The radius of the core has implications for possible mantle convection scenarios and in particular for the presence of a perovskite phase transition at the bottom of the mantle, which enables global plume-like features to exist and persist over time(Spohn et al., 1998). Such strong, long-standing mantle plumes arising from the core-mantle boundary may explain the long-term volcanic activity in the Tharsis area. Nevertheless, their existence during the last billion years is uncertain. An alternative scenario is that the thermal insulation by locally thickened crust, which has a lower thermal conductivity and is enriched in radioactive elements in comparison to the mantle, leads to significant lateral temperature variations in the upper mantle that are sufficient to generate partial melt even today (Schumacher and Breuer, 2006). We note that the tidal Q of Mars is ~80 (Smith and Born, 1976), substantially less than that of the Earth’s mantle (~200), despite it being smaller and presumably cooler than the Earth.
A geophysical “snapshot” of Mars should reveal at a minimum the basic radial compositional structure: dimensions and properties of the crust, the upper and lower mantle, and the solid and/or liquid core. It should also place strong constraints on the radial thermal structure. Studies undertaken during the past decade have developed joint inversion strategies using multiple data sets (e.g., geodetic, seismic and EM; Verhoeven et al., 2005). These methods have been successfully applied to recover the temperature, mineralogy, and iron content of the Earth's lower mantle without trade-off between structural parameters (Verhoeven et al., 2009). The compositional structure relates to the bulk composition of the planet and early differentiation and fractionation of the interior, a time when life may have been spawned on Mars. Thermal structure is derived from the radial seismic velocity structure (particularly phase boundaries), heat flow, and EM sounding and provides the “end condition” on thermal evolution scenarios. Whereas much insight can be gained from a few representative measurements, the delineation of lateral variations in mantle thermal structure derived from a geophysical network with an adequate distribution of stationsis necessary to gain full appreciation of heat transfer processes. It is very likely that there remain strong thermal anomalies in the mantle and spatial variations in lithospheric thickness from hot spot processes. In fact without this lateral information the average radial geophysical properties may not be well determined (Kiefer and Li, 2009).
The four primary methods for geophysically probing a planet’s interior from its surface are seismology, heat flow, EM sounding, and precision tracking (for rotation measurements), with seismology being by far the most powerful of these. Each is discussed in more detail below.
Seismology
Seismology has provided detailed interior models for both the Earth (with dense networks) and on the Moon (with a sparse network), the latter rangingfrom simple spherical models (Nakamura, 1983; Gagnepain-Beyneix et al., 2006) to more complex models dealing with the lateral variations (Chenet et al., 2006; Zhao et al., 2008). The level of martian seismic activity remains unknownbecause of the high sensitivity to wind and poor coupling to the ground of the deck-mounted Viking seismometer (Goins and Lazarewicz, 1979; Nakamura and Anderson, 1979). From models of the thermoelastic cooling of the lithosphere and extrapolation from visible faults (Phillips, 1991; Golombek et al., 1992; Knapmeyer et al., 2006), seismic activity about 100 times higher than that on the Moon has been estimated. A medium activity model (Knapmeyer et al., 2006) generates about 100 quakes/yr with seismic moment greater than 1014 Nm (magnitude Mw = 3.3) and one per year of seismic moment greater than 1017 Nm (Mw = 5.3). A concentration of seismicity in the Tharsis bulge is suggested by analysis of visible tectonic faults (Knapmeyer et al., 2006). Impacts are an additional seismic source which may occur at a rate similar to that of the Moon (Davis, 1993). As likely seismic properties of Mars have also been studied quite extensively in the last two decades (Lognonné et al., 1996; Sohl and Spohn, 1997; Mocquet, 1999;Gudkovaand Zharkov, 2004; Lognonné and Johnson, 2007), rather strong and conservative constraints can be used for estimating the amplitude of seismic waves, leading to two possible levels of seismic instrumentation (see Table). Levels 0 and 1 (L0 and L1) provide two basic specifications in terms of the quality of the seismometer installation.
Level 0 / 3-axis VBB (0.02 Hz5 Hz: <109 ms2/Hz1/2)Z-axis SP (0.1 Hz50 Hz: < 108 ms2/Hz1/2) / Mass: 4-5 kg (3.5-4 kg for instrument, 0.5-1 kg for deployment); Deployment: installed inside the lander
Level 1 / 3-axis VBB (1 mHz5 Hz: < 1010 ms2/Hz1/2)
Z-axis SP (0.1 Hz50 Hz: < 108 ms2/Hz1/2) / Add: pressure sensor (1 mHz5 Hz; 103 Pa/Hz1/2), wind/thermal shield; Additional Mass: ~2.5 kg + arm mass; Deployment: installed directly on the ground
Table: Two levels of seismometer installation. Masses are examples from ExoMars Phase B and include I/F, and maturity margins. VBB–very broad band, SP–short period, Z–vertical. Level 0 corresponds to a medium-noise installation and Level 1 represents a better installation with ultra-low-noise instruments.
Seismic network requirements
In order to fully reach their scientific goals,seismic investigations will require a network of at least four L1 stations: three with a spacing of about 3000 km (~50°), and an antipodal station capable of detecting seismic waves traveling through the core (e.g., PKP) from an event simultaneously detected by the others. Such a network may locate, through travel-time analysis, more than 80 globally detectable quakes per (Earth) year andwill be robust to unexpectedly high mantle attenuation or low seismic activity. With four or more landers, details of the internal structure, such as the dichotomy or other large unit differences, mantle discontinuities and anisotropy, may be characterized. A less effective L0 network might expect to locate about 20 quakes per year and must therefore last for at least one martian year to be of significant value.